An interesting matter seems to be the lance types available in jousts and their breakability. According to this
source in French (Loïs Forster,
Joust, most gracious of war art), it seems there are/were three of them (which certainly have an impact on... actual knights' impact) :
1) the
war lance (called lances à outrance in French during jousts) : strong with a sharp metal tip,
2) the "
lance à plaisance", less dangerous, easier to break (made with "un fer émoussé (non affûté) ou munies d’un rochet, embout formé généralement d’une triple pointe métallique, divisant d’autant la puissance d’impact en chaque point de contact et favorisant le travail de résistance de l’armure, tout en diminuant les risques de glissement")
(Btw don't tell Henri II of France breaking a lance during a joust is not dangerous),
3) the
modern lance used in jousts practiced nowadays as a sport or a show with tips made of balsa (a fragile wood) and pre-split, even easier to break.
The first two types were used in medieval/renaissance jousts. The "lance à plaisance" is said to be posterior to the 13th century AD, as an option to make jousts safer.
It seems that, at least during a period of time and maybe only in some areas, one of the criteria of victory in jousts was the number of lances you could break on your opponents as the
Tiptoft Rules (1466) shows it and also Froissart, in the late XVth century, about the Irish knighting ceremony mentioned in the
article whose link was posted above :
“The young aspirant has to joust with light lances, such as he can easily hold, against a shield set up in a meadow on a post. The more lances he breaks, the greater the honor for him.”
That said, "a very common attested practice was to choose thick lances, hard to break, showing a willingness more to impress and show his worth than to score points" (Loïs Forster,
Joust, most gracious of war art).
Maybe interestingly, a reference French dictionary of 1694 says :"In jousts, a "lance brisée" (a "broken lance") is a lance half-sawn next to the tip, so that it can easily break".
This raises the questions of the breakability of the war lance on the battlefield, of our ability to assess their actual impact/force in war conditions, etc. If the lance isn't meant to break at impact, the shock will be more violent for both opponents.